Microbeam analysis -- Analytical electron microscopy -- Method for the determination of interface position in the cross-sectional image of the layered materials

ISO 20263:2017 specifies a procedure for the determination of averaged interface position between two different layered materials recorded in the cross-sectional image of the multi-layered materials. It is not intended to determine the simulated interface of the multi-layered materials expected through the multi-slice simulation (MSS) method. This document is applicable to the cross-sectional images of the multi-layered materials recorded by using a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM) and the cross-sectional elemental mapping images by using an energy dispersive X-ray spectrometer (EDS) or an electron energy loss spectrometer (EELS). This document is also applicable to the digitized image recorded on an image sensor built into a digital camera, a digital memory set in the PC or an imaging plate and the digitalized image converted from an analogue image recorded on the photographic film by an image scanner.

Analyse par microfaisceaux -- Microscopie électronique analytique -- Méthode de détermination de la position d'interface dans l'image de coupe transversale des matériaux en couches

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Publication Date
30-Nov-2017
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6060 - International Standard published
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20-Oct-2017
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INTERNATIONAL ISO
STANDARD 20263
First edition
2017-11
Microbeam analysis — Analytical
electron microscopy — Method for the
determination of interface position
in the cross-sectional image of the
layered materials
Analyse par microfaisceaux — Microscopie électronique analytique
— Méthode de détermination de la position d'interface dans l'image
de coupe transversale des matériaux en couches
Reference number
ISO 20263:2017(E)
ISO 2017
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ISO 20263:2017(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2017, Published in Switzerland

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ii © ISO 2017 – All rights reserved
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ISO 20263:2017(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms, definitions and abbreviated terms ................................................................................................................................ 1

3.1 Terms and definitions ....................................................................................................................................................................... 1

3.2 Abbreviated terms ............................................................................................................................................................................... 4

4 Specimen preparation for cross-sectional imaging ......................................................................................................... 4

4.1 General ........................................................................................................................................................................................................... 4

4.2 Requirements for the cross-sectional specimen ........................................................................................................ 5

5 Determination of an interface position ........................................................................................................................................ 6

5.1 General ........................................................................................................................................................................................................... 6

5.2 Preliminary considerations .......................................................................................................................................................... 6

5.2.1 Ideal model of an interface ...................................................................................................................................... 6

5.2.2 More realistic model of an interface ............................................................................................................... 6

5.2.3 Dealing with intensity fluctuations in the image ................................................................................. 8

6 Detailed procedure for determining the position of the interface ...................................................................8

6.1 General ........................................................................................................................................................................................................... 8

6.2 Preparing cross-sectional TEM/STEM image ............................................................................................................10

6.2.1 Preparing digitized Image ....................................................................................................................................10

6.2.2 Displaying the digitized image ..........................................................................................................................11

6.3 Setting the ROI ......................................................................................................................................................................................11

6.3.1 General...................................................................................................................................................................................11

6.3.2 Classification of image .............................................................................................................................................11

6.3.3 Procedure of setting the ROI ...............................................................................................................................12

6.4 Acquisition of the averaged intensity profile .............................................................................................................17

6.5 Moving-averaged processing ...................................................................................................................................................19

6.6 Differential processing ..................................................................................................................................................................20

6.7 Final location of the interface ..................................................................................................................................................21

7 Uncertainty ..............................................................................................................................................................................................................22

7.1 Uncertainty accumulating from each step of the procedure ........................................................................22

7.2 Uncertainty of measurement result on image analysis .....................................................................................22

Annex A (informative) Examples of processing the real TEM/STEM images for three

image types .............................................................................................................................................................................................................24

Annex B (informative) Two main applications for this method ............................................................................................36

Annex C (informative) Calibration of scale unit: Pixel size calibration .........................................................................43

Bibliography .............................................................................................................................................................................................................................45

© ISO 2017 – All rights reserved iii
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ISO 20263:2017(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www.iso.org/patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following

URL: https://www.iso.org/iso/foreword.html.

This document was prepared by Technical Committee ISO/TC 202, Microbeam analysis, Subcommittee

SC 3, Analytical electron microscopy.
iv © ISO 2017 – All rights reserved
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ISO 20263:2017(E)
Introduction

Multi-layered materials are widely used in the production of semiconductor devices, various kinds

of sensors, coating films for optical element, new functional materials, etc. One of the factors used

to determine the characteristics of multi-layered materials is the layer thickness, for evaluation of

products and verification of the production process. In practice, measuring the total thickness and/or

the thickness of each layer and checking the uniformity of thickness and/or flatness of the interface are

often done using recorded images of the materials. Evaluations can be made from the cross-sectional

TEM/STEM images by accurately determining the averaged interface position between two different

layered materials.

In relation to the determination of the interface position in the HR atomic imaging, analysis by the

multi-slice simulation (MSS) method can be applied for the target measurement, if the atomic structural

models can be constructed. However, in real materials, there are a lot of cases when they cannot, as

follows:

— the interface between amorphous layers, or layers of amorphous substance and crystal;

— the interface recorded in low-resolution image in which the atomic columns cannot be identified: 1)

very thick single-layered material, 2) thick multi-layered material.

This document relates the method to determine the averaged interface position, using a differential

processing of the accumulated intensity profile getting from the ROI set in the cross-sectional

TEM/STEM image of the multi-layered materials. The thickness of the layer that can be applied ranges

from a few nanometers to a few micrometers. Thus, this document is not intended for the determination

of the simulated position of the layer interface analysed by the MSS method.
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INTERNATIONAL STANDARD ISO 20263:2017(E)
Microbeam analysis — Analytical electron microscopy —
Method for the determination of interface position in the
cross-sectional image of the layered materials
1 Scope

This document specifies a procedure for the determination of averaged interface position between two

different layered materials recorded in the cross-sectional image of the multi-layered materials. It is

not intended to determine the simulated interface of the multi-layered materials expected through the

multi-slice simulation (MSS) method. This document is applicable to the cross-sectional images of the

multi-layered materials recorded by using a transmission electron microscope (TEM) or a scanning

transmission electron microscope (STEM) and the cross-sectional elemental mapping images by using

an energy dispersive X-ray spectrometer (EDS) or an electron energy loss spectrometer (EELS). This

document is also applicable to the digitized image recorded on an image sensor built into a digital

camera, a digital memory set in the PC or an imaging plate and the digitalized image converted from an

analogue image recorded on the photographic film by an image scanner.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at http://www.electropedia.org/
3.1.1
atomic column image

TEM/STEM image recorded at atomic-resolution from a specimen along a high-symmetry crystalline

orientation

Note 1 to entry: Crystalline orientation is the direction of crystal which is represented by Miller indices. During

TEM imaging, it is often useful to have a crystalline specimen aligned so that a specific (low index) zone axis

(3.1.26) is parallel, or near parallel, to the beam direction (optical axis).
3.1.2
cross-sectional image

TEM/STEM image of the multi-layered materials along a plane perpendicular to the stacking direction

3.1.3
differential processing

calculation of the difference between the values of adjacent pixel data in the intensity profile

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ISO 20263:2017(E)
3.1.4
digital camera

device that detects the image using a chip-arrayed image sensor (3.1.12), such as a charge-coupled

device (CCD) or complementary metal-oxide semiconductor (CMOS), which converts a visual image to

an electric signal
[SOURCE: ISO 29301:2010, 3.8]
3.1.5
elemental mapping image

image produced by the selected signal which is attributed to a particular element, from the EDS/EELS

spectrum
3.1.6
FIB thinning

site-specific thinning technique using abrasion by focused field-emitted gallium atoms accelerated to

an energy of 1 keV to 40 keV to thin a particular region of the specimen
3.1.7
filtering mask
mask to define the cut–off frequency in the reciprocal space
3.1.8
fast Fourier transformation
FFT
efficient algorithm to compute the discrete Fourier transform
[SOURCE: ISO 15932:2013, 5.4.1.1]
3.1.9
inverse fast Fourier transformation
IFFT
efficient algorithm to compute the inverse of the discrete Fourier transform
[SOURCE: ISO 15932:2013, 5.4.1.2]
3.1.10
image file format
format for saving an image as a computer file according to a predetermined rule
3.1.11
image scanner

device that converts an analogue image into a digitized image with the desired resolution

Note 1 to entry: There are mainly two different types of scanners: flatbed type and drum type.

[SOURCE: ISO 29301:2010, 3.18, modified — the example has been added.]
3.1.12
image sensor

device, such as a charge-coupled device (CCD) array or complementary metal-oxide semiconductor

(CMOS) sensor, which converts visual image information to an electric signal, built-in digital camera

(3.1.4) or other imaging devices
3.1.13
intensity profile
signal intensity distribution along a line specified in the image
3.1.14
interface

boundary surface at the junction of two different layers of materials recorded in the cross-sectional

image (3.1.2) of the multi-layered materials
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ISO 20263:2017(E)
3.1.15
ion milling
thinning technique of sputtering the specimen with an inert gas
[SOURCE: ISO 15932:2013, 4.1]
3.1.16
imaging plate

electron image detector consisting of a film with a thin active layer embedded with specifically designed

phosphors[SOURCE: ISO 29301:2010, 3.17]
[SOURCE: ISO 29301:2010, 3.23]
3.1.17
low pass filter
filter to pass signals of frequencies lower than the cut-off frequency
3.1.18
moving average

calculation for averaging the selected dataset which is picked out from equal number of dataset on

either side of a central data
3.1.19
multi-slice simulation
multi-slice method
MSS

computer simulation method of high-resolution TEM images, which treats electrons as incoming waves

and treats the interactions with matter as occurring on multiple successive single slices of the specimen

[SOURCE: ISO 15932:2013, 6.4.1, modified — “algorithm for the simulation” has been replaced by

“computer simulation method”.]
3.1.20
multi-layered material

laminated material which is fabricated by alternating layers of at least two kinds of materials on the

substrate
3.1.21
photographic film

sheet or a roll of thin plastic coated by photographic emulsion for recording an image

[SOURCE: ISO 29301:2010, 3.26]
3.1.22
pixel
smallest unit element that makes up the digital image
3.1.23
pixel-resolution
number of imaging pixels (3.1.22) per unit distance of the detector

Note 1 to entry: Typical unit of measurement is “pixels per unit distance”, e.g. dots per inch (dpi).

[SOURCE: ISO 29301:2010, 3.27, modified — Note 1 to entry has been added.]
3.1.24
region of interest
ROI
sub-dataset picked out from the entire dataset for a specific purpose
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ISO 20263:2017(E)
3.1.25
ultra-microtome

thin sectioning instrument to prepare the specimen thin enough for TEM observation by using glass or

diamond knives
3.1.26
zone axis

crystallographic direction, designated [u v w], defined by the intersection of a number of crystal planes

(h ,k ,l …….h ,k ,l ) such that all of the planes satisfy the so-called Weiss zone law; hu + kv + lx = 0

1 1 1 i i i
[SOURCE: ISO 29301:2010, 3.38]
3.2 Abbreviated terms
AEM Analytical electron microscope/microscopy
CCD Charge coupled device
CRT Cathode ray tube
EDS Energy dispersive X-ray spectrometer/spectroscopy
EDX Energy dispersive X-ray spectrometer/spectroscopy
EELS Electron energy loss spectrometer/spectroscopy
FFT Fast Fourier transformation
FIB Focused ion beam
HREM High-resolution transmission electron microscope/microscopy
IFFT Inverse fast Fourier transformation
MSS Multi-slice simulation
ROI Region of interest
STEM Scanning transmission electron microscope/microscopy
TEM Transmission electron microscope/microscopy
4 Specimen preparation for cross-sectional imaging
4.1 General

To determine the interface potion of the multi-layered materials stacked on a substrate, the specimen

observed by TEM/STEM shall be cut into a cross-sectional thin slice perpendicular to the stacking

direction of the multi-layered thin film, using the techniques of ultra-microtome, ion-milling, FIB

thinning, chemical etching and so on. In order to keep the thickness information of the layered materials

with an accuracy of 1 %, cut out angle α [shown in Figure 1, a)] shall be 90 ± 6 degrees.

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ISO 20263:2017(E)
a) Multi-layered specimen b) TEM/STEM observation
c) Cross-sectional imaging
Key
1 substrate
2 multi-layered materials
3 direction of electron beam
4 thin slice of the specimen
5 cross-sectional TEM/STEM image
6 arrows indicate interface positions
Figure 1 — Specimen preparation for cross-sectional imaging
4.2 Requirements for the cross-sectional specimen
Ensure that the specimen

— provides a good contrast and clear interface for the multi-layered materials in the TEM/STEM/

elemental mapping image,

— can be cleaned to remove contamination without causing mechanical/electrical damage or

distortion,

— has a smooth surface on both sides and identical thickness, at least within the area used for the

determination process of interface position,

— is aligned to a low-index zone axis along the electron optical axis, if the specimen region is a single

crystal.
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ISO 20263:2017(E)
5 Determination of an interface position
5.1 General

It is important to determine the position of an interface in a layered material from its cross-sectional

TEM/STEM/elemental mapping image, objectively and uniquely. In this clause, the main scheme for the

determination of the interface position, as prescribed by this document, is explained.

5.2 Preliminary considerations
5.2.1 Ideal model of an interface

Ideally, the interface between two kinds of materials, M and M , show straight edge [Figure 2 a)]. In

1 2

this case, it is easy to find the interface positions (S and S ) uniquely from the intensity profile [see

1 2
Figure 2 b)] along a line (L) perpendicular to the interface.
a) Ideal interface (S and S) model between two kinds of layers, M and M
1 2 1 2

b) Intensity profile along an arrow line, L, in a), perpendicular to the interfaces (S and S )

1 2
Figure 2 — Ideal interface model
5.2.2 More realistic model of an interface

However, in general, the interface will not be in a straight line. It is a region with gradated intensity

distribution between layers M and M [see Figure 3 a)]. In this case, it is not easy to find the accurate

1 2

interface position from its intensity profile which is normally s-shaped [see Figure 3 b)]. This document

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ISO 20263:2017(E)

defines the interface position at the steepest tilt angle in the slope. Differential processing of the

intensity profile is the most suitable method to determine the interface position as defined above.

Figure 3 c) shows the differential curve of the intensity profile on in Figure 3 b). Pixel positions on the

x-axis corresponding to the minimal value and maximal value in the curve show the interface positions

(S and S ) on both sides of the layer M .
1 2 2
a) Realistic interface (S and S) model between two kinds of layers, M and M
1 2 1 2

b) Intensity profile along an arrow line, L, in a), perpendicular to the interfaces (S and S )

1 2
c) Differential curve of intensity profile
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ISO 20263:2017(E)

NOTE Positions of minimal and maximal value correspond to the interfaces (S and S ) defined in this

1 2
document.
Figure 3 — Realistic interface model
5.2.3 Dealing with intensity fluctuations in the image

Unlike models described in 5.2.1 and 5.2.2, the actual cross-sectional TEM/STEM/elemental mapping

image has a domain-like intensity fluctuation, background noise and sometimes (in the high-resolution

images) periodic modulation of the intensity due to atomic column structures. Because of this non-

uniformity in the intensity in the image, follow the steps a) to f) sequentially for obtaining the desired

smooth intensity profile with a plateau and well-defined slope.
NOTE 1 Details of the actual procedure are described in Clause 6.
a) Prepare the cross-sectional TEM/STEM/elemental mapping digital image.
Set the direction of the interface parallel to the y-axis of the monitor screen.
b) Set the ROI area in the image.

c) Average the intensity line profile, perpendicular to the interface (parallel to the x-axis of the

monitor screen) along the interface (parallel to the y-axis of the monitor screen) in the ROI area.

d) Apply “moving-average” processing to the averaged intensity profile produced by the previous step,

c). This will remove small noise from the boundary region contributing to the slope of the interface.

e) Apply differential processing to the resulting intensity profile obtained in d) by the moving-average

process.

f) Determine the interface position as the pixel coordinate on the x-axis which either corresponds to

the maximal or minimal value in the differential curve.
NOTE 2 More details follow in Clause 6.
6 Detailed procedure for determining the position of the interface
6.1 General

As described in the previous clause, the interface position can be determined through the differential

processing of the intensity profile in the ROI.

In the differential processing, a noise component existing in the intensity profile becomes an obstacle

to find the correct interface position. Therefore, it is necessary to remove the noise components in

advance through the average processing and the moving-averaged processing.

Also, in atomic resolution image with an atomic column arrangement along the interface, the oscillated

component depends on the atomic column cannot be eliminated even in the averaged intensity profile.

This is an obstacle to the extraction of the correct interface position by differential processing.

Therefore, for such image, pre-processing for obscuring the atomic column structure is essential

through the processing of FFT/low pass filtering/IFFT.

Figure 4 shows a flow chart of the interface position determination procedure described in 5.2.3. In this

clause, details of each procedure will be described.
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ISO 20263:2017(E)
Figure 4 — Flow chart of interface position determination procedure
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ISO 20263:2017(E)
6.2 Preparing cross-sectional TEM/STEM image
6.2.1 Preparing digitized Image

It is necessary to prepare a digitized cross-sectional image to determine the interface position. The bit

depth of digitization of the image shall be larger than 8 bits. There are four ways of digitizing the image

corresponding to each image detection system shown in Table 1.
Table 1 — Comparison table for image detection
Image detection
Apparatus for digitization Pixel size
system

Photographic film Flatbed image scanner Determined by resolution applied to image scanner

Imaging plate Dedicated scanner Determined by LASER beam diameter for readout
Image sensor
Built in the digital camera Same size as that of the image sensor
(Digital camera)
Built in the PC connected to
Digital memory Determined by scanning condition of electron beam
the scanning device

a) Photographic film: Analogue image recorded on the photographic negative film shall be converted

to a digitized image by using image scanner with resolution more than 1 200 dpi.

NOTE 1 Use a flatbed image scanner because it is easy to set the transparent scale in it for pixel size

calibration (Refer to ISO 29301).

b) Imaging plate (IP): The recorded image shall be read out with dedicated image digitizer (IP reader)

connected to a PC.

c) Image sensor: The digitized image, captured by a digital camera, shall be saved on the memory in

the PC system as an image file with a reversible format.

NOTE 2 Ensure that the procedure for normalization of gain is performed to have a uniform background

of the image.

d) Digital memory: STEM and elemental mapping image, captured by the digital memory built into a

PC, shall be saved as an image file with a reversible format.

Before and during the execution of the digitization procedure, ensure the following.

— The correct sensitivity setting is used for the photographic film used, to generate the negative image

with proper density and contrast.

— Keep the exposure time to minimize to drift and reduce blurring in the recorded image.

— Do not use “binning” in the readout process of the magnified digital image from the digital camera.

— When using an image montage function, ensure that the seams of the image do not overlap with any

of the interface of the specimen.

— For saving the digitized images, an uncompressed image file format (ESP, PICT, TIFF or Windows

bitmap), or reversible (lossless) compressed image file format (GIF or PING) shall be used.

— Ethical digital imaging requires that the original uncompressed image file be stored on archival

media, e.g. CD-R, without any image manipulation or processing operation. All parameters of the

production and acquisition of this file and any subsequent processing steps shall be documented

and reported to ensure reproducibility.

Generally, acceptable imaging operations include gamma correction, histogram stretching and

brightness and contrast adjustments need not be reported. All other operations (such as Unsharp-

Masking, Gaussian Blur, etc.) shall be directly identified by the author as part of the experimental

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ISO 20263:2017(E)

methodology. However, for diffraction data or any other image data that is used for subsequent

quantification, all imaging operations shall be reported.
6.2.2 Displaying the digitized image

The digitized image used for further processing shall be orientated so that the interface is, as far as

possible, parallel to the y-axis of the monitor screen. If the interface is tilted with an angle, α, to the

y-axis of the monitor screen, it is necessary to measure and to re
...

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